Photomixing is the generation of continuous wave

terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of fre ...

from two lasers. The beams are mixed together and focused onto a photomixer device which generates the terahertz radiation. It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources,

quantum cascade laser Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, ...

and

ultrashort pulse In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by m ...

d lasers with photoconductive switches as used in

terahertz time-domain spectroscopy In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on ...

. The advantages of this technique are that it is continuously tunable over the frequency range from 300 GHz to 3 THz (10 cm⁻¹ to 100 cm⁻¹) (1 mm to 0.1 mm), and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10⁻⁸ W.

Principle

Two continuous wave

lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

with identical polarisation are required, the lasers with

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

ω₁ and ω₂ are spatially overlapped to generate a terahertz beatnote. The co-linear lasers are then used to illuminate an ultra fast semiconductor material such as

GaAs Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circ ...

. The photonic absorption and the short charge carrier lifetime results in the modulation of the conductivity at the desired terahertz frequency ω_THz = ω₁ - ω₂. An applied electric field allows the conductivity variation to be converted into a current which is radiated by a pair of antenna. A typical photoconductive device or 'photomixer' is made from low temperature

with a patterned metalized layer which is used to form an electrode array and radiating antenna.

High resolution spectrometer

The photomixing source can then form the basis of a laser spectrometer which can be used to examine the THz signature of various subjects such as gases, liquids or solid materials. The instrument can be divided into the following functional units: * Laser sources which provide a THz beatnote in the optical domain. These are usually two near infrared lasers and maybe an optical amplifier. * The photomixer device converts the beatnote into THz radiation, often emitted into free space by an integrated antenna. * A THz propagation path, depending on the application suitable focusing elements are used to

collimate A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction ...

the THz beam and allow it to pass through the sample under study. * Detector, with the relatively low levels of available power, in the order of 1 µW, a sensitive detector is required to ensure a reasonable signal to noise ratio. Si bolometers provide a solution for in-coherent instruments. Alternatively a second photomixer device can be used as a detector and has the advantage of allowing coherent detection.

References

Francis Hindle, Arnaud Cuisset, Robin Bocquet, Gaël Mouret "Continuous-wave terahertz by photomixing: applications to gas phase pollutant detection and quantification" Comptes Rendus Physique (2007), {{doi, 10.1016/j.crhy.2007.07.009 Electromagnetic spectrum Terahertz technology